This invention relates to defrosting control devices, and more particularly, to an improved frost sensor used to control the defrost cycle of a refrigeration device. While the invention is described with particular emphasis on air conditioning applications, those skilled in the art will recognize the wider applicability of the inventive concepts disclosed hereinafter.
Refrigeration systems, particularly the evaporator coils of air conditioning equipment and the like, are susceptible to frost accumulation during operation. Frost accumulation is undesirable in that it decreases the cooling efficiency of the system, and raises operating costs. Even though its deficiencies are well known, frost accumulation and the related problem of detection have plagued refrigeration systems for a long time, and a number of methods and devices have been proposed to control removal of the frost. For example, timers often are employed to initiate a defrost cycle after some elapsed time period. Mechanical counters have been utilized in certain domestic appliances to initiate a defrost cycle after some preset number of door openings on the appliance. Photoelectric ice detecting devices also have been proposed for frost detecting purposes. The photoelectric detecting devices in general operate by causing radiant energy from a light source to be reflected, scattered, refracted or otherwise transmitted from the radiant energy source to a sensor, the amount of radiant energy transmitted in turn being dependent upon the frost or ice build-up on a particular surface.
While these prior art devices work well for their intended purposes, they all may be characterized as operating on symptoms of frost rather than the true frosting condition of the evaporator coil. That is to say, prior art devices have not provided detectors which are capable of close simulation of the frosting characteristics or conditions in the evaporator coil that the frost detector was intended to sense. Consequently, prior art sensors often initiate defrost cycles when that cycle is not needed, a procedure known in the art as a "false" defrost cycle. False defrost cycles decrease system efficiency, which tends to defeat the very purpose of the frost detector. Although the invention disclosed hereinafter employs a photocell as the actual sensing device, the total sensor construction is distinguishable from prior art frost detectors employing photocell devices in that the sensor of the invention accumulates frost exactly as the evaporator coil accumulates frost so that the defrosting cycle is initiated only when required for efficient evaporator operation. In addition, the same detector that initiates the defrost cycle also is used to terminate the defrost cycle. The elimination of false defrost cycles in the refrigeration system results in increased overall system efficiency and decreased operating costs. Use of the same detector to terminate defrost ensures that the defrost cycle actually is completed before normal operation is again undertaken.
One of the objects of this invention is to provide an improved frost sensor for detecting the presence of frost.
Another object of this invention is to provide a frost sensor in which the formation of frost is analogous to the formation of frost in the refrigeration unit with which the sensor finds application.
Another object of this invention is to provide a low-cost frost detector for a refrigeration unit.
Still another object of this invention is to provide a frost detector in which factors affecting frost accumulation in the sensor may be varied.
Another object of this invention is to provide a detector for a refrigeration unit that controls both the activation and de-activation of a defrost cycle for the system.
Another object of this invention is to provide a frost detector having an air passageway through it which uses the pressure drop across the evaporator coil of a refrigeration system to draw air through the passageway.
Other objects will be apparent to those skilled in the art in light of the following description and accompanying drawings.